The study, in the April issue of the Journal of the American Dietetic Association, found that twice as many teens and nearly double the number of young adults who had been vegetarians reported having used unhealthy means to control their weight, compared with those who had never been vegetarians. Those means included using diet pills, laxatives and diuretics and inducing vomiting to control weight.

There's a dark side to vegetarianism, said Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine. He had no role in the research.

"Adolescent vegetarians [in the study] were more prone to disordered eating and outright eating disorders," Katz said. "This is not due to vegetarianism but the other wayaround: Adolescents struggling to control their diets and weight might opt for vegetarianism among other, less-healthful efforts."

Vegetarianism, or a mostly plant-based diet, can be recommended to all adolescents, Katz said. "But when adolescents opt for vegetarianism on their own, it is important to find out why because it may signal a cry for help, rather than the pursuit of health," he said.

Katz said he thinks a balanced vegetarian diet is among the most healthful of dietary patterns, and the study suggests some of the benefits.

"Adolescents practicing vegetarianism were less likely to be overweight than their omnivorous counterparts and, were the measures available, would likely have had better blood pressure and cholesterol, too," he said. "Eating mostly plants -- and even only plants -- is good for us, and certainly far better for health than the typical American diet."

The study's lead researcher, Ramona Robinson-O'Brien, an assistant professor in the Nutrition Department at the College of Saint Benedict and Saint John's University in St. Joseph, Minn., agreed.

"The majority of adolescents and young adults today would benefit from improvements in dietary intake," she said. The study found, for instance, that the vegetarians among the participants generally were less likely to be overweight or obese.

"However, current vegetarians may be at increased risk for binge eating, while former vegetarians may be at increased risk for extreme unhealthful weight-control behaviors," she said. "Clinicians and nutrition professionals providing guidance to young vegetarians might consider the potential benefits associated with a healthful vegetarian diet, [but should] recognize the possibility of increased risk of disordered eating behaviors."

The researchers collected data on 2,516 teens and young adults who participated in a study called Project EAT-II: Eating Among Teens. They classified participants as current, former or never vegetarians and divided them into two age groups: teens (15 to 18) and young adults (19-23).

Each participant was questioned about binge eating, whether they felt a loss of control of their eating habits and whether they used any extreme weight-control behaviors.

About 21% of teens who had been vegetarians said they used unhealthy weight-control behaviors, compared with 10% of teens who had never been vegetarians. Among young adults, more former vegetarians (27%) had used such measures than current vegetarians (16%) or those who'd never been vegetarians (15%), the study found.

In addition, among teenagers, binge eating and loss of control over eating habits was reported by 21% of current and 16% of former vegetarians but only 4% of those who'd never followed a vegetarian diet. For young adults, more vegetarians (18%) said they engaged in binge eating with loss of control than did former vegetarians (9%) and those who were never vegetarians (5%), the study found.

Young adult vegetarians were less likely to be overweight or obese than were those who'd never been vegetarians. Among teens, the study found no statistically significant differences in weight.

"When guiding adolescent and young adult vegetarians in proper nutrition and meal planning, it is important to recognize the potential health benefits and risks associated with a vegetarian diet," Robinson-O'Brien said. "Furthermore, it may be beneficial to investigate an individual's motives for choosing a vegetarian diet and ask about their current and former vegetarian status when assessing risk for disordered eating behaviors."

SOURCES: Ramona Robinson-O'Brien, Ph.D., R.D., assistant professor, Nutrition Department, College of Saint Benedict and Saint John's University, St. Joseph, Minn.; David L. Katz, M.D., M.P.H., director, Prevention Research Center, Yale University School of Medicine, New Haven, Conn.; April 2009 Journal of the American Dietetic Association